The circuit for energizing and operating the rapid start fluorescent lamp in wide use is well-known. The majority of such lamps are rated at 40 watts and are dimensioned
as T-12, that is, 11/2 inches in diameter and 48 inches long. Each lamp has a filament of robust construction at each end which is energized continuously. The filaments aid in starting by producing clouds of electrons at the ends of the lamp and contribute to the reliability of the lamp operation.
The 40 watt T-12 rapid start lamp ignites at about 200 volts R.M.S. and operates at about 102 volts R.M.S. The ballast for energizing and operating two lamps is conventionally constructed to apply about 300 volts to each lamp seriatim and revert from a starting circuit to an operating circuit automatically.
Because each lamp has two pins at each end it was heretofore deemed that the particular circuit which is practically universally used was safe to humans who might be inserting or changing lamps while the circuit was energized. It was considered that if the voltage to ground through a lamp, that is, with one end held by a person and the other end in a socket, did not exceed 180 volts R.M.S. or 325 volts peak, the person would not be subjected to shock hazard.
Tests by Underwriters' Laboratories have now raised some doubt that the conventional rapid-start fluorescent lamp circuit is totally safe when one end of a lamp is grasped by a person and the opposite end inserted into a socket. This assumes that the circuit is energized. As a result U.L. has required that until the lamp has been firmly seated in its sockets there be no current flow exceeding 5 m.a. and that there be no likelihood that the lamp ignite.
This problem of ensuring the safety of rapid-start lamps has been considered in prior circuitry disclosed in U.S. Pat. Nos. 2,767,349 and 3,040,208. None of the circuits of these prior art patents taught how to utilize disconnect sockets in a two-lamp series circuitry of the type where there is a single primary and a single secondary connected in economical autotransformer arrangement.
The invention achieves the required safety against shock hazard by the use of novel circuits including disconnects, this being achieved with great economy. Economy is an essential requirement because the slightest increase in basic cost of a circuit is elevated in the eventual cost to the user.
Until the invention herein it was not deemed economically possible to use disconnect sockets in a two-lamp rapid start fluorescent lamp circuit while preserving the autotransformer connection of transformer windings. The addition of disconnect sockets to a two-lamp rapid start fluorescent lamp circuit could only be accomplished by isolating the primary and secondary windings and making the connections somewhat like those used in a known instant start circuit (U.S. Pat. No. 2,552,111). This would require a complete rearrangement of the ballast circuitry and thus a radical departure from the economical manufacture of the modern two-lamp rapid start fluorescent ballast of today. Voltages to ground would be different as well as starting aid voltages. Industry requirements of today could not be met.
As will be seen from the specification herein the invention achieves the required safety without radically changing the construction of the two-lamps rapid start fluorescent lamp ballast thereby enabling the same methods of manufacture to be used. The circuitry is varied by miniscule changes in connecting the ballasts in the fixtures and/or during installation into connection with a line. All voltages to ground are preserved.